Manufacturing method for composite gear

Abstract

To excellently ensure bonding strength between a first gear and a second gear forming a composite gear, while simplifying a manufacturing process of the composite gear.SOLUTION: A manufacturing method for a composite gear combines a first gear and a second gear, and involves bringing the first and second gears into contact with each other and bonding them by diffusion bonding. The manufacturing method eliminates the need to weld the first gear and second gear that form the composite gear at multiple locations, so that it is possible to excellently ensure bonding strength between the first gear and second gear that form the composite gear while simplifying a manufacturing process of the composite gear.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a compound gear that combines a first gear and a second gear. [Background technology]

[0002] Conventionally, a double-helical gear has been known in which a first gear and a second gear, each having teeth inclined in opposite directions relative to the axial direction, are arranged axially on a rotating shaft (see, for example, Patent Document 1). The first gear of this double-helical gear includes a first annular portion whose outer periphery is integrally formed with teeth and whose inner periphery is fitted to the rotating shaft, and both axial ends of the first annular portion are joined (welded) to the rotating shaft by laser welding or electron beam welding. That is, a first weld is formed on the axial end face of the first annular portion opposite the second gear by axially welding the fitting portion between the inner periphery of the first annular portion and the rotating shaft. Furthermore, a second weld is formed in the axial gap between the teeth of the first gear and the teeth of the second gear by welding the end face of the first annular portion facing the second gear to the rotating shaft from the radially outer side. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-066004 Summary of the Invention [Problem to be solved by the invention]

[0004] In the conventional double-helical gears described above, multiple welds must be formed to ensure sufficient joint strength between the first and second gears. However, welding the first and second gears at multiple locations can complicate the manufacturing process and increase costs.

[0005] Therefore, a main object of the present disclosure is to simplify the manufacturing process of a compound gear while ensuring good joint strength between the first gear and the second gear that form the compound gear. [Means for solving the problem]

[0006] The method of manufacturing a compound gear disclosed herein is a method of manufacturing a compound gear that combines a first gear and a second gear, in which the first and second gears are brought into contact with each other and joined by diffusion bonding.

[0007] This method allows the newly formed surfaces of the first gear and the second gear to come into contact with each other and combine the separate crystals into a single crystal through atomic diffusion, thereby firmly joining the first and second gears. As a result, there is no need to weld the first gear and second gear that form the compound gear at multiple locations, which simplifies the manufacturing process of the compound gear while ensuring good joint strength between the first gear and second gear that form the compound gear. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing a compound gear manufactured by the manufacturing method of the present disclosure. [Figure 2] FIG. 1 is a cross-sectional view showing a compound gear manufactured by the manufacturing method of the present disclosure. [Figure 3] 1 is a flowchart for explaining a manufacturing method of a compound gear according to the present disclosure. [Figure 4] 10 is a flowchart illustrating a modified embodiment of the method for manufacturing a compound gear according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0009] Next, embodiments of the present disclosure will be described with reference to the drawings.

[0010] Fig. 1 is a perspective view of a compound gear 10 manufactured by the manufacturing method of the present disclosure, and Fig. 2 is a cross-sectional view of the compound gear 10. The compound gear G0 shown in these drawings is a double helical gear that combines a first gear 11, which is a helical gear, with a second gear 12, which is a helical gear whose twist direction is set in the opposite direction to that of the first gear 11. The first gear 11 has a plurality of teeth 11t and an inner hole 11o, each of which has a tooth trace inclined relative to the axis of the first gear 11. The second gear 12 has a plurality of teeth 12t and an inner hole 12o, each of which has a tooth trace inclined relative to the axis of the second gear 12 and extends in the opposite direction to the tooth trace of the teeth 11t of the first gear 11.

[0011] 2, a ring-shaped recess 115 is formed on the flat end face 110 on the inner side (on the second gear 12 side) of the first gear 11, with its center at the axis of the first gear 11. Furthermore, a cylindrical protrusion 125 is formed on the flat end face 120 on the inner side (on the first gear 11 side) of the second gear 12, with its center at the axis of the second gear 12. The protrusion 125 has an outer diameter slightly larger than the inner diameter of the recess 115 of the first gear 11, and a height slightly larger than the depth of the recess 115. As a result, by applying pressure to the first and second gears 11, 12 and pressing the convex portion 125 into the recess 115, the end face of the convex portion 125 of the second gear 12 is brought into close contact with the bottom surface of the recess 115 of the first gear 11, and the outer surface of the convex portion 125 of the second gear 12 is brought into close contact with the inner surface of the recess of the first gear 11, and further, the end face 110 of the first gear 11 (the portion surrounding the recess 115) and the end face 120 of the second gear 12 (the portion surrounding the convex portion 125) can be brought into close contact.

[0012] Next, a method for manufacturing the above-mentioned compound gear 10 will be specifically described.

[0013] In manufacturing the compound gear 10, a base steel, such as case hardening steel containing at least one of Cr and Mo, is subjected to rough forging (step S101), gear cutting (step S111), and semi-finishing (step S121) to manufacture a first gear 11 before surface heat treatment, finishing, or tooth finishing (honing). The first gear 11 obtained through steps S101, S111, and S121 has an annular recess 115 formed on its inner end face 110. Separately from the first gear 11, a second gear 12 is manufactured using the same base steel as the first gear 11, and is subjected to rough forging (step S102), gear cutting (step S112), and semi-finishing (step S122) to manufacture a second gear 12 before surface heat treatment, finishing, or tooth finishing (honing). The second gear 12 obtained by the processing of steps S102, S112 and S122 has a cylindrical protrusion 125 formed on the inner end face 120.

[0014] The steel used for the first and second gears 11, 12 may contain, for example, 0.17% by mass or more and 0.23% by mass or less of C (carbon), 0.15% by mass or more and 0.35% by mass or less of Si (silicon), 0.55% by mass or more and 0.95% by mass or less of Mn (manganese), 0.030% by mass or less of P (phosphorus), 0.030% by mass or less of S (sulfur), 0.25% by mass or less of Ni (nickel), 0.85% by mass or more and 1.25% by mass or less of Cr (chromium), 0.15% by mass or more and 0.30% by mass or less of Mo (molybdenum), 0.30% by mass or less of Cu (copper), and inevitable impurities. Alternatively, case hardening steel such as JIS-SCM420 may be used as the steel used for the first and second gears 11, 12.

[0015] The first gear 11 manufactured in steps S101-S121 and the second gear 12 manufactured in steps S102-S122 are set one by one in a pressure device (not shown). Then, the first and second gears 11, 12 are aligned so that the phases of the teeth 11t and 12t match, and then a predetermined pressure is applied to the first and second gears 11, 12 by the pressure device so that the protrusion 125 of the second gear 12 is press-fit into the recess 115 of the first gear 11 (step S130). This integrates the first and second gears 11, 12, so that the end face of the convex portion 125 of the second gear 12 is in close contact with the bottom surface of the concave portion 115 of the first gear 11, the outer surface of the convex portion 125 of the second gear 12 is in close contact with the inner surface of the concave portion of the first gear 11, and further, the end face 110 of the first gear 11 and the end face 120 of the second gear 12 are in close contact with each other.

[0016] Next, the integrated first and second gears 11 and 12 are subjected to a bonding and carburizing process (step S140) in a vacuum furnace (not shown). During the bonding and carburizing process, the pressure inside the vacuum furnace is set to a target pressure of, for example, 2 kPa or less, and the internal temperature of the vacuum furnace is set to a target temperature within a range of, for example, 900-1050°C. The first and second gears 11 and 12 are heated in the vacuum furnace, which is essentially in a vacuum state. This causes oxygen in oxides present at the contact surfaces between the first gear 11 and the second gear 12, i.e., the end face 110 of the first gear 11, the inner and bottom surfaces of the recessed portion 115, the end face 120 of the second gear 12, and the end and outer surfaces of the protruding portion 125, to evaporate. As a result, the newly formed surfaces of the first and second gears 11 and 12 come into contact with each other, atomic diffusion progresses, and the separate crystals merge into a single crystal, firmly bonding the first and second gears 11 and 12 together through diffusion bonding.

[0017] After the pressure and temperature in the vacuum furnace reach the target pressure and target temperature, and a predetermined joining time has elapsed by which joining of the first and second gears 11, 12 is considered complete, a hydrocarbon gas (e.g., acetylene gas) is introduced into the vacuum furnace while maintaining the pressure and temperature in the vacuum furnace at the target pressure or target temperature. This causes C (carbon) to penetrate and diffuse into the joined first and second gears 11, 12 through so-called vacuum carburization. The introduction of the hydrocarbon gas into the vacuum furnace is stopped when the predetermined carburization time has elapsed since the start of introduction.

[0018] After the joining and carburizing process in step S140 is completed, the vacuum and heating in the vacuum furnace are stopped, and the joined first and second gears 11 and 12 are quenched in the vacuum furnace (step S150). In this embodiment, a reservoir tank for storing a quenching coolant such as water or oil is installed in the vacuum furnace. After the joining and carburizing process is completed, the joined first and second gears 11 and 12 are transferred into the reservoir tank by a transfer mechanism (not shown). After quenching is completed, the first and second gears 11 and 12 are removed from the vacuum furnace and subjected to finishing (step S160) and tooth finishing (honing, step S170). This results in a compound gear 10 with high strength and high dimensional accuracy.

[0019] As described above, the compound gear 10 is formed by bringing the first and second gears 11, 12 into contact with each other and joining them by diffusion bonding. That is, diffusion bonding brings newly formed surfaces of the first gear 11 and the second gear 12 into contact with each other and combines separate crystals into a single crystal through atomic diffusion, thereby firmly joining the first and second gears 11, 12. As a result, there is no need to weld the first gear 11 and the second gear 12 that form the compound gear 10 at multiple locations, which simplifies the manufacturing process for the compound gear 10 and ensures good bonding strength between the first gear 11 and the second gear 12 that form the compound gear 10.

[0020] In the above embodiment, the first and second gears 11, 12 are formed so that one is press-fitted into the other, and after one of the first and second gears 11, 12 is press-fitted into the other (step S130), the first and second gears 11, 12 are heated in a reduced-pressure furnace (step S140). This allows the first and second gears 11, 12 to be maintained in contact with each other, i.e., under pressure, in the reduced-pressure furnace without using a jig or the like, further simplifying the manufacturing process of the compound gear 10.

[0021] Furthermore, in the above embodiment, after the first and second gears 11, 12 are heated in a reduced-pressure furnace for a predetermined joining time, the first and second gears 11, 12 are subjected to a carburizing process by introducing a hydrocarbon gas (carbon) into the reduced-pressure furnace while continuing to heat them. This allows the first and second gears 11, 12 to be joined and carburized by heating them once in the reduced-pressure furnace. As a result, it is possible to ensure the joining strength and surface hardness of the first and second gears 11, 12 while preventing a decrease in strength of the first and second gears 11, 12 due to multiple heating (tempering) processes. In addition, in the above embodiment, the first and second gears 11, 12 that have been subjected to the carburizing process are quenched in the reduced-pressure furnace (step S150). This further simplifies the manufacturing process of the compound gear 10 and further improves the strength of the compound gear 10.

[0022] 3, nitriding may be performed instead of carburizing. In this case, in step S140, a predetermined joining time, during which joining of the first and second gears 11 and 12 is considered complete, may be elapsed after the pressure and temperature in the vacuum furnace reach a target pressure (e.g., 2 kPa or less) and a target temperature (800-850°C), respectively. Then, ammonia gas may be introduced into the vacuum furnace while maintaining the pressure and temperature at the target pressure or temperature. This allows nitrogen (N) to penetrate and diffuse (form a solid solution) into the joined first and second gears 11 and 12 by so-called vacuum nitriding. Therefore, similar to the case of carburizing, joining of the first and second gears 11 and 12 and nitriding can be completed by heating the first and second gears 11 and 12 once in the vacuum furnace. As a result, it is possible to prevent the strength of the first and second gears 11, 12 from decreasing due to multiple heating (tempering), while ensuring good joint strength and surface hardness of the first and second gears 11, 12. The introduction of ammonia gas into the reduced pressure furnace may be stopped when a predetermined nitriding time has elapsed since the start of the introduction.

[0023] In step S140, a nitriding process may be performed after the carburizing process. In this case, in step S140, once the pressure in the vacuum furnace reaches a target pressure (e.g., 2 kPa or less) and the temperature reaches a first target temperature (e.g., 900-1050°C) and the bonding time has elapsed, a hydrocarbon gas (e.g., acetylene gas) is introduced into the vacuum furnace while maintaining the pressure and temperature at the target pressure or target temperature. Furthermore, once a predetermined carburizing time has elapsed since the start of the introduction of the hydrocarbon gas, the introduction of the hydrocarbon gas is stopped, and the temperature in the vacuum furnace is lowered to a second target temperature (e.g., 800-850°C) lower than the first target temperature. After the temperature in the vacuum furnace reaches the second target temperature, ammonia gas is introduced into the vacuum furnace. Furthermore, once a predetermined nitriding time has elapsed since the start of the introduction of the ammonia gas, the introduction of the ammonia gas is stopped.

[0024] As a result, carbon (C) and nitrogen (N) penetrate and diffuse (dissolve) into the bonded first and second gears 11, 12 at temperatures appropriate for vacuum carburizing and vacuum nitriding, respectively. Therefore, the bonding, carburizing, and nitriding of the first and second gears 11, 12 can be completed by heating the first and second gears 11, 12 once to the first target temperature in a reduced-pressure furnace and then cooling them to the second target temperature. This ensures the bond strength and surface hardness of the first and second gears 11, 12 while preventing a decrease in strength due to multiple heating (tempering). Furthermore, because the temperatures of the first and second gears 11, 12 are lowered to at least the second target temperature before the hardening process in step S150, deformation of the first and second gears 11, 12, i.e., the compound gear 10, due to the hardening process can be effectively prevented.

[0025] FIG. 4 is a flowchart illustrating a modified embodiment of the method for manufacturing the compound gear 10 of the present disclosure.

[0026] 4, similar to the process shown in Fig. 3, a first gear 11 is manufactured by rough forging (step S101), gear cutting (step S111), and semi-finishing (step S121) using a steel material such as case hardening steel containing at least one of Cr and Mo. The first gear 11 is then manufactured before surface heat treatment, finishing, or tooth finishing (honing). The second gear 12 is manufactured separately from the first gear 11 by rough forging (step S102), gear cutting (step S112), and semi-finishing (step S122) using the same steel material as the first gear 11. The second gear 12 is then manufactured before surface heat treatment, finishing, or tooth finishing (honing). Furthermore, the first gear 11 manufactured in steps S101-S121 and the second gear 12 manufactured in steps S102-S122 are set in a pressure device not shown, and the aligned first and second gears 11, 12 are pressurized to pressurize the convex portion 125 of the second gear 12 into the concave portion 115 of the first gear 11 (step S130).

[0027] Next, the integrated first and second gears 11 and 12 undergo a bonding and carburizing cooling process (step S145) in a vacuum furnace (not shown). During the bonding and carburizing cooling process, the pressure inside the vacuum furnace is set to a target pressure of, for example, 2 kPa or less, and the internal temperature of the vacuum furnace is set to a target temperature within a range of, for example, 900-1050°C. The first and second gears 11 and 12 are heated in the vacuum furnace, which is essentially in a vacuum state, causing oxygen in oxides present at the contact surfaces between the first gear 11 and the second gear 12 to evaporate. As a result, the newly formed surfaces of the first and second gears 11 and 12 come into contact with each other, atomic diffusion progresses, and the separate crystals merge into a single crystal, firmly bonding the first and second gears 11 and 12 together through diffusion bonding.

[0028] After the pressure and temperature in the vacuum furnace reach the target pressure and target temperature, and a predetermined joining time has elapsed by which joining of the first and second gears 11, 12 is considered complete, a hydrocarbon gas (e.g., acetylene gas) is introduced into the vacuum furnace while maintaining the pressure and temperature at the target pressure or target temperature (step S145). This allows carbon (C) to penetrate and diffuse into the joined first and second gears 11, 12 through so-called vacuum carburization. The first and second gears 11, 12 are heated in the vacuum furnace for a predetermined carburization time (e.g., 60-300 minutes) while supplying the hydrocarbon gas, so that the carbon concentration in the region corresponding to the surface of the compound gear 10 (first and second gears 11, 12) becomes greater than 0.5% by mass and less than 0.9% by mass, or greater than 1.0% by mass and less than 1.5% by mass. The surface layer of compound gear 10 is a region extending to a depth of approximately 0.15 to 0.45 mm from the outer peripheral surface, including the tooth flanks, tip surfaces, and root surfaces (outer peripheral surfaces) of teeth 11t, 12t. The introduction of hydrocarbon gas into the reduced-pressure furnace is stopped when the carburizing time has elapsed from the start of the introduction, and when the carburizing treatment (heating) is completed, almost the entire first and second gears 11, 12 have an austenite structure.

[0029] After the carburizing process (introduction of hydrocarbon gas) is completed, the first and second gears 11, 12 are cooled in a reduced-pressure furnace (step S145). This cooling process involves gradually cooling the first and second gears 11, 12 at a cooling rate (e.g., 0.2-5.0°C / s) below the critical cooling rate at which the austenite structure of the first and second gears 11, 12 transforms to martensite. By slowly cooling the first and second gears 11, 12 in this manner, the outer and inner peripheral surfaces of the first and second gears 11, 12 are primarily pearlite, with the ferrite structure increasing toward the center. This prevents the martensite structure, which has a larger volume than the pearlite structure, from forming in the first and second gears 11, 12, thereby suppressing distortion of the first and second gears 11, 12 due to the heat treatment.

[0030] After the cooling process is complete, the first and second gears 11, 12 are removed from the vacuum furnace and hardened outside the vacuum furnace (step S155). In step S155, the first and second gears 11, 12 are hardened by high-frequency induction heating. After hardening is complete, the first and second gears 11, 12 are finished (step S160) and then honed (step S170). This completes the compound gear 10, which has high strength and dimensional accuracy.

[0031] 4 , manufacturing compound gear 10 by the process shown in Fig. 4 eliminates the need to weld the first gear 11 and second gear 12 that form compound gear 10 at multiple locations, simplifying the manufacturing process for compound gear 10 while ensuring good joint strength between first gear 11 and second gear 12 that form compound gear 10. Also, by press-fitting one of the first and second gears 11, 12 into the other in step S130, the first and second gears 11, 12 can be maintained in contact with each other, i.e., under pressure, in a reduced-pressure furnace without using a jig or the like, further simplifying the manufacturing process for compound gear 10.

[0032] Furthermore, the carburizing treatment in step S145 is performed so that the C concentration in the surface layers of the compound gear 10 (first and second gears 11, 12) becomes higher than the C concentration of the base steel, to more than 0.5 mass% and not more than 0.9 mass%, or more than 1.0 mass% and not more than 1.5 mass%. The cooling treatment in step S145 gradually cools the first and second gears 11, 12 in a reduced-pressure furnace at a cooling rate that is lower than the critical cooling rate at which the austenitic structure of the first and second gears 11, 12 transforms to martensite.

[0033] In this way, by slowly cooling the first and second gears 11, 12 in a reduced-pressure furnace at a cooling rate below the critical cooling rate after carburizing (heating), the outer and inner peripheral surfaces of the first and second gears 11, 12 are primarily pearlite, with the ferrite structure increasing toward the center. This prevents martensite, which has a larger volume than pearlite, from forming in the first and second gears 11, 12, thereby preventing distortion of the first and second gears 11, 12 during subsequent heat treatment. Therefore, by hardening the cooled (slowly cooled) first and second gears 11, 12 using high-frequency induction heating, it is possible to further improve the strength of the compound gear 10 while preventing distortion of the first and second gears 11, 12.

[0034] Note that, in step S145 of FIG. 4 , the above-described nitriding treatment may be performed instead of the carburizing treatment. This makes it possible to ensure good joint strength and surface hardness of the first and second gears 11, 12 while preventing a decrease in strength of the first and second gears 11, 12 due to multiple heating (tempering). Furthermore, in step S145 of FIG. 4 , nitriding treatment may be performed after the carburizing treatment. This makes it possible to ensure good joint strength and surface hardness of the first and second gears 11, 12 while preventing a decrease in strength of the first and second gears 11, 12 due to multiple heating (tempering), and effectively suppress deformation of the first and second gears 11, 12, i.e., the compound gear 10, due to the quenching treatment in step S155.

[0035] Alternatively, the quenching treatment in step S155 may involve heating the first and second gears 11 and 12, which have been subjected to the joining and carburizing cooling treatment (step S145), in a heating furnace (so-called continuous furnace) controlled so that the internal temperature reaches a predetermined target temperature Ttag, and then cooling at a cooling rate equal to or greater than the critical cooling rate. More specifically, the target temperature Ttag is set to a constant value (e.g., 800°C) equal to or greater than the Acm transformation point, which is the austenitizing temperature corresponding to the carbon concentration in the surface layer of the compound gear 10, and is within a range of up to the A3 transformation point + 100°C. The first and second gears 11 and 12 are then heated for a predetermined time (e.g., 30 to 120 minutes) in a heating furnace (not shown) in which the internal temperature is maintained within a range of the target temperature Ttag ±5°C and the carbon potential (CP) is set to, for example, CP = 0.8. This makes it possible to suppress variation in the degree to which carbides precipitated at the grain boundaries of the first and second gears 11, 12 dissolve into the grains, while making the first and second gears 11, 12 almost entirely of an austenitic structure.

[0036] Furthermore, when the C concentration in the surface layer of the compound gear 10 is greater than 1.0 mass% and less than or equal to 1.5 mass%, setting the target furnace temperature Ttag within the range of 770-890°C increases the amount of carbide at the grain boundaries and reduces the amount of retained austenite in the surface layer. Therefore, setting the target furnace temperature Ttag within the range of 770-890°C increases the hardness of the surface layer, thereby further improving the wear resistance of the compound gear 10. However, when the C concentration in the surface layer of the compound gear 10 is greater than 1.0 mass% and less than or equal to 1.5 mass%, the target furnace temperature Ttag may be set to a temperature above 890°C. This allows the first and second gears 11 and 12, which have been quenched at a higher temperature, to be rapidly cooled, thereby incorporating more carbon into the grains, strengthening the grains, and increasing the amount of retained austenite in the surface layer, thereby further improving the toughness of the compound gear 10.

[0037] Furthermore, if the first and second gears 11, 12 are heated for a predetermined time and then rapidly cooled (quenched) in a heating furnace using a refrigerant such as water or cooling oil at 20-200°C at a cooling rate equal to or greater than the critical cooling rate, a portion of the austenite structure in the regions along the outer and inner peripheral surfaces of the first and second gears 11, 12 changes to martensite (quenched martensite). This results in the regions along the outer and inner peripheral surfaces of the first and second gears 11, 12 becoming harder than the base steel. Furthermore, by setting the refrigerant temperature to 20-200°C, it is possible to reliably prevent an increase in the volume fraction of retained austenite that is not transformed into martensite, even if the C concentration in the regions of the first and second gears 11, 12 corresponding to the surface layers of the compound gear 10 is greater than 1.1% by mass and less than 1.5% by mass.

[0038] Furthermore, although the compound gear 10 is a double helical gear formed by joining the first and second gears 11, 12 so that their helix directions are opposite to each other, the present invention is not limited to this. That is, the compound gear 10 may be a helical gear formed by joining the first and second gears 11, 12 so that their helix directions are the same. Furthermore, the compound gear 10 may be a two-stage gear formed by joining the first and second gears 11, 12, both of which are spur gears.

[0039] The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the present disclosure. Furthermore, the above-described embodiment is merely a specific form of the invention described in the Summary of the Invention, and does not limit the elements of the invention described in the Summary of the Invention. [Industrial Applicability]

[0040] The invention of the present disclosure can be used in the compound gear manufacturing industry and the like. [Explanation of symbols]

[0041] 10 compound gear, 11 first gear, 110 end surface, 115 recess, 12 second gear, 120 end surface, 125 protrusion.

Claims

1. A method for manufacturing a compound gear comprising a first gear and a second gear, a manufacturing method for a compound gear, the first and second gears being brought into contact with each other and joined by diffusion bonding;

2. 2. The method for manufacturing a compound gear according to claim 1, the first and second gears are formed so that one is press-fitted into the other; a manufacturing method for a compound gear, wherein one of the first and second gears is press-fitted into the other, and then the first and second gears are heated in a reduced pressure furnace.

3. 3. The method for manufacturing a compound gear according to claim 1 or 2, the first and second gears are heated in the reduced pressure furnace for a predetermined time, and then carbon or nitrogen is introduced into the reduced pressure furnace while continuing to heat the first and second gears, thereby carburizing or nitriding the first and second gears.

4. 4. The method for manufacturing a compound gear according to claim 3, the first and second gears that have been subjected to the carburizing treatment or the nitriding treatment are hardened in the reduced pressure furnace.

5. 3. The method for manufacturing a compound gear according to claim 1 or 2, the first and second gears are heated in the reduced pressure furnace for a predetermined time, carbon is introduced into the reduced pressure furnace while the heating is continued to subject the first and second gears to a carburizing treatment, the temperature inside the reduced pressure furnace is lowered, and nitrogen is introduced into the reduced pressure furnace to subject the first and second gears to a nitriding treatment.

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